1. Field of the Invention
The present invention relates to a solid-state imaging device, and more particularly to a solid-state imaging device based on CMOS technology.
2. Description of the Related Art
The CMOS-based solid-state imaging device, generally an active pixel sensor or a CMOS image sensor, has within each pixel of a pixel array a photoelectric convertor and transistors which reset and amplify the convertor, and has, in peripheral circuitry, drive circuits which drive the individual pixels and also output circuits and image processing circuits which amplify electrical signals output from the individual pixels and carry out image processing. It is anticipated that such CMOS image sensors will provide solid-state imaging devices that are less costly on account of the CMOS processes used in fabrication and that are capable of achieving a higher image quality due to improvements in pixel structure.
Because CMOS image sensors are used in camera modules installed on mobile devices such as cellular phones and information terminals, there is a strong desire in the industry for larger scale integration and a smaller chip size. In order to satisfy this aspect a multilevel metallization structure within the chip is required. However, the use of a multilevel metallization structure means the formation over the pixels of a multilevel metallization structure composed of multiple metallization levels and dielectric layers between the levels, making it difficult to efficiently guide incident light to the photoelectric convertors within the pixels. Decreased sensitivity and blurring arise as a consequence of the scattering of incident light by the multilevel metallization and attenuation of the incident light by the multiple layers of dielectric film.
FIG. 1 is a schematic cross-sectional diagram of a prior-art CMOS image sensor. A chip 1 consisting of a semiconductor substrate has at the center thereof a pixel array 10 composed of a plurality of pixels, and peripheral circuitry 12 is formed at the perimeter of the pixel array 10. Lower and upper layers of multilevel metallization 22 and 20 are formed on the substrate over both the pixel array 10 and the peripheral circuitry 12.
The CMOS image sensors currently in common use typically employ a multilevel metallization structure having, for example, three levels. In addition to the peripheral circuitry, this multilevel metallization is used also for the connection of transistors within the pixels. However, as noted above, because it is necessary to avoid scattering incident light, Japanese Patent Application Laid-open No. 2004-221527 and Japanese Patent Application Laid-open No. 2003-249632 describe the use of a structure in which metallization is not formed over the photoelectric convertor within the pixels and which is able guide incident light to the convertor. According to this prior art, a trench-like hole is formed in the multilevel metallization structure over each pixel, and a dielectric material is buried in the hole to give a photoconductive structure.
In addition, the CMOS image sensor has over each pixel a microlens for collecting incident light and a color filter. Such a construction requires a complex fabrication process. As a solution to this problem, Japanese Patent Application Laid-open No. 2002-134726, for example, teaches a process in which part of the dielectric film over the pixel is removed to form a trench, and a color filter pattern having a convex shape is formed thereon to serve as both the microlens and the color filter.